Sorry to say I disagree with this. Somewhere on this forum there's a post talking about similar. The members kid would only ride a superman (iirc) adorned bike, of which he apologised for.
Now, there's certainly no need to apologise. Any type of bike, adorned with whatever it may be, that gets a kid interested in riding, is a good bike imvho.
My first bike was a Raleigh Buster which I loved because it was cowboy themed, which I was totally into. Very fond memories of riding that bike up and down the road.
Our boy's three and a half and will be getting a first pedal bike for Christmas. I'm currently on the lookout for something second hand in decent nick. I might look out for one of those Specializeds, the Frogs and Islabikes are a lot of money, even used.
Our daughter's seven and has just never got on with cycling. Never took to the balance bike, we've tried her on a pedal bike which she didn't like so we fitted stabilisers, that didn't work so I've taken the cranks off so she can scoot it but she's really not interested in learning. She's good on her scooter though so we'll be upgrading that soon.